Solid state electrochemical lithium/polymer cell

ABSTRACT

A lithium electrochemical cell is provided that provides many cycles  with loss in capacity. The cell contains an ion conducting solid polymer electrolyte and an electronically conductive, anion-intercalating polymer cathode.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalty thereon.

FIELD OF INVENTION

The invention relates in general to lithium electrochemical cells and inparticular to solid state electrochemical lithium/polymer cells thatprovide many cycles without a loss in capacity.

BACKGROUND OF THE INVENTION

Two major areas of consideration in the development of lithiumelectrochemical cells are to find a liquid electrolyte that does notreact with metallic lithium and to identify a lithium anionintercalating cathode as for example Li_(x) CoO₂, Li_(x) TiS₂, andLi_(x) Mn₂ O₄ that provides many cycles without loss in capacity. Theabsence of a liquid component would reduce or eliminate corrosion of themetallic lithium anode, as well as harmful chemical reactions at thecathode.

SUMMARY OF THE INVENTION

The general object of this invention is to provide an improved lithiumelectrochemical cell. A more particular object of the invention is toprovide such a cell that does not contain a liquid component that cancause corrosion of the metallic lithium anode as well as harmfulchemical reactions at the cathode. A still further object of theinvention is to provide such a lithium electrochemical cell that allowsfor high cycle life and good capacity retention. Another object of theinvention is to provide such a lithium electrochemical cell that can beused as either a primary cell or a rechargeable cell.

It has now been found that the aforementioned objects can be attained bymaking a solid state lithium electrochemical cell that contains an ionconducting solid polymer electrolyte and an electronically conductive,anion intercalating polymer cathode.

Absence of a liquid component reduces or eliminates corrosion of themetallic lithium anode, as well as harmful chemical reactions at thecathode. At the cathode, anions are intercalated rather than cations,with anion-doped polymer being electrically conductive, and undopedpolymer being electrically resistive. The polymer cathode is somewhatflexible, allowing anion doping and undoping to occur without structureddamage, allowing for high cycle life and good capacity retention.

The use of the solid state lithium electrochemical cells as rechargeablecells allows for the development of thin film rechargeable mono andbipolar lithium cells. The novel features of the cell include a systemthat is completely solid state. Moreover, polymers are employed as bothcathode and electrolyte. Then, too, anions (instead of lithium cations)are shuttled in and out of the cathode. The polymeric electrolyte andcathode components total only a few microns in thickness. This allowsfor the manufacture of a very thin, multicell, high voltage battery.

Because the system is solid state, any desired geometric form ispossible, and battery packages can be sealed in metallized plasticfilms. This reduces packaging weight considerably compared to theheavy-walled metal cans normally used for cells containing liquidelectrolyte.

A solid electrolyte according to the invention includes a polymer hostcontaining dissolved lithium salt, allowing the movement of Li⁺ and X⁻ions between the electrodes. There are many advantages associated withthe use of ionically conducting polymers as for example (poly (ethyleneoxide)) such as high speed processing of thin, light weight bipolarcells. Solid polymer electrolyte (SPE) films can act both as amechanical separator between the anode and cathode, and as abinder/adhesive to insure contact between electrodes. Elasticity allowsthe SPE to conform to electrode volume changes during cycling. Further,safety is enhanced in that there is no liquid electrolyte to leak fromcells. The SPE that can be used in the invention include at least onelithium salt selected from the group consisting of LiClO₄, LiBF₄,LiAsF₆, LiCF₃ So₃, and LiN(CF₃ So₂)₂ dissolved in at least one polymerhost selected from the group consisting of poly (ethylene oxide),poly(propylene oxide) poly(dioxolane) and poly (methylmethacrylate).

Electropolymerized films on the order of one micron in thickness can beprepared to serve as the cathode material, that, when doped with anionson charging, are electronically conductive. Upon discharge, anionsreturn to the electrolyte. Suitable solid polymer cathodes that can beused in the invention include poly(3-methylthirophene)poly(alkyl-thiophene) poly(aniline) poly(acetylene), or mixturesthereof.

The invention provides a solid state lithium electrochemical system thatutilizes thin, solid polymer films as electrolyte and cathode, and thatincludes an anode of either metallic lithium or a lithium intercalatingmaterial. Such lithium intercalating materials include lithium alloy,LiC₆, lithiated graphite, and lithiated petroleum coke.

An all-solid state lithium electrochemical cell is made with anionically conducting SPE and an electronically conductive polymercathode. The SPE is comprised of lithium salts (LiCF₃ SO₃, LiN(CF₃SO₂)₂) dissolved in poly(ethylene oxide) (PEO). The cathode ispoly(3-methylthiophene) (PMT). In this cell, the polymer cathode acceptsanions common to the SPE on charge, and releases anions into the SPE ondischarge. Likewise, lithium ions from the SPE are plated as metalliclithium during charge, and released to the SPE on discharge as seen inthe following equation: ##STR1##

Since anions and cations are simply being shuttled between electrodes,no new products are formed during charge or discharge, so no chemicalreactions detrimental to cycle life occur. Therefore, exceptional cyclelife and capacity are seen.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Electrochemical polymerization of poly(3-methylthiophene) (PMT) films isaccomplished in a 125 ml European flask using a 1 cm² platinum flagcounter electrode, a saturated sodium calomel reference electrode, and aplatinum rod working electrode. One end of the working electrode ispolished to a mirror finish with an alumina/water paste and sheathed inheat shrinkable Teflon, exposing a 0.071 cm² surface area. The cell isflooded with a solution containing 0.1M 3-methylthiophene monomerdissolved in acetonitrile that also contains a 0.1M concentration ofeither tetrabutylammonium tetrafluoroborate, LiCF₃ SO₃, or LiN[(CF₃SO₂)]₂. Ultra high purity argon is bubbled through the electrolyte toremove any oxygen. The PMT films, 1.4 microns thick, are polymerized at10 mA cm⁻² by passing a charge of 0.25 C cm⁻² five (1.25 C cm⁻² total),with five minute periods at open circuit between depositions. The PMT-covered electrode is rinsed in acetonitrile and dried under vacuum at50° C. Polymerized films contain anions corresponding to the salt used:BF₄ ⁻, CF₃ SO₃ ⁻, N(CF₃ SO₂)₂ ⁻.

PEO(MW=4×10⁶), LiCF₃ SO₃, and LiN(CF₃ SO₂)₂ in a molar ratio of 20:1:1are dissolved in acetonitrile that has been distilled while bubbling dryargon to form a viscous solution. PMT-covered electrodes are fitted witha small lithium metal reference electrode, then dipped into the polymersolution four or five times. Between each dip, films are allowed tostand to permit the CH₃ CN to evaporate, leaving a solid polymerelectrolyte covering the PMT and reference electrodes. Finishedelectrodes are dried overnight under active vacuum at 50° C. Laboratorycells are constructed by pressing the electrodes against metalliclithium (anodes) and maintaining slight pressure to ensure mechanicalcontact. Cell cycling is performed galvanostatically with an EG&G PARModels 173 potentiostat/galvanostat controlled by a HP86B computer.

EXAMPLE 1

The electrochemical cell or system is:

    Li/(PEO).sub.20 (LiCF.sub.3 SO.sub.3).sub.1, (LiN[CF.sub.3 S).sub.2 ].sub.2).sub.1 /PMT-BF.sub.4.sup.-

In this cell, PMT is polymerized with BF₄ as the dopant anion. The cellis discharged at 5 μA cm⁻² to a 2.0 V cutoff, and charged at 2.5 μA to a3.8 V cutoff for 170 cycles at a temperature between 19° C. and 22° C.Good retention of capacity is observed and mean cycling efficiency is98.4%. Initial load potential is above 3.6 V with voltage graduallydecreasing until near the end of discharge when voltage drags abruptlyfrom about 3.0 V to the 2.0 V cutoff. There is an increase in capacityover the first few cycles that is explained by an increase in the levelof polymer doping as the PMT becomes better able to accommodate anions.

EXAMPLE 2

This electrochemical system or cell is:

    Li/(PEO).sub.20 (LiCF.sub.3 SO.sub.3).sub.1 (LiN[CF.sub.3 SO.sub.2 ].sub.2).sub.1 /PMT-CF.sub.3 SO.sub.3.sup.-

In this cell, the PMT dopant anion (CF₃ SO₃ ⁻) introduced duringelectropolymerization is common to anions present in the electrolyte.Discharge (5 μA cm⁻² to 2.0 V) and charge (3.5 μA cm² to 3.8 V) capacityincreases over the first few cycles from 2 mA h g⁻¹ to 3.5 mA h g⁻¹until cycle 19, where capacity drops to 1.2 mA h g⁻¹. Thereafter,capacity is very consistent, fading to slightly above 1.0 mA h g⁻¹ bycycle 87.

EXAMPLE 3

The electrochemical cell or system is:

    Li/(PEO).sub.20 (LiCF.sub.3 SO.sub.3).sub.1 (LiN[CF.sub.3 SO.sub.2 ].sub.2).sub.1 /PMT-N(CF.sub.3 SO.sub.2).sub.2.sup.-

In this instance, the imide anion (N(CF₃ SO₂)₂ ⁻) initially doped intothe PMT is also common to the electrolyte. Discharge and charge are bothat a rate of 5 μA cm⁻² between voltage cutoffs of 2.0 V and 3.8 Vrespectively. Capacity increases over the first few cycles to 12.9 mA hg⁻¹. At cycle 25, capacity falls to 6.6 mA h g⁻¹ then recovers slightlyto remain between 7.5 to 8.5 mA h g⁻¹ through cycle 72.

I wish it to be understood that I do not desire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. A solid state electrochemical cell includingmetallic lithium as an anode, (PEO)₂₀ (LiCF₃ SO₃)₁ (LiN[CF₃ SO₂ ]) as asolid polymer electrolyte and and poly (3-methylthiophene as a solidpolymer cathode.
 2. A solid state electrochemical cell includingmetallic lithium as an anode, (PEO)₂₀ (LiCF₃ SO₃)₁ (LiN[CF₃ SO₂ ]₂)₁ asa solid polymer electrolyte and poly 3-methylthiophene-BF₄ as a solidpolymer cathode.
 3. A solid state electrochemical cell includingmetallic lithium, as an anode, PEO)₂₀ (LiCF₃ SO₃)₁ (LiN[CF₃ SO₂ ]₂)₁ asa solid polymer electrolyte and poly(3-methylthiophene-CF₃ SO₃ as solidpolymer cathode.
 4. A solid state electrochemical cell includingmetallic lithium as an anode, (PEO)₂₀ (LICF₃ SO₃)₁ (LiN[CF₃ SO₂ ]₂)₁ asa solid polymer electrolyte, and poly(3-methylthiophene)-N(CF₃ SO₂)₂ asa solid polymer cathode.